Electrostatographic or xerographic copiers, printers and digital imaging systems typically record an electrostatic latent image on an imaging member. The latent image corresponds to the informational areas contained within a document being reproduced. In one type of such a system, a uniform charge is placed on a photoconductive member and portions of the photoconductive member are discharged by a scanning laser or other light source to create the latent image. The latent image is then developed by bringing a developer, including colorants, such as, for example, toner particles, into contact with the latent image. The toner particles carry a charge and are attracted away from a toner supply and toward the latent image by an electrostatic field related to the latent image, thereby forming a toner image on the imaging member. The toner image is subsequently transferred to a physical media, such as a print sheet. The print sheet, having the toner image thereon, is then advanced to a fusing station for permanently affixing the toner image to the print sheet.
In multi-color electrophotographic printing, multiple latent images corresponding to each color separation are recorded on one or more photoconductive surfaces. The electrostatic latent image for each color separation is developed with toner of that color. Thereafter, each color separation is ultimately transferred to the print sheet in superimposed registration with the other toner images, creating, for example, a multi-layered toner image on the print sheet. This multi-layer toner image is permanently affixed to the print sheet to form a finished print.
In any printing apparatus, it is desirable to set up a feedback system by which the quality of output prints is monitored, and the behavior of the apparatus is monitored to counteract any detected print defects. U.S. Published Patent Application 2007/0003302 describes an extensive feedback system, wherein images (test images, or images such as those to be printed) are recorded in detail from the imaging surface of a photoreceptor, using input scanning hardware comparable in resolution and quality to that used for recording hard-copy images in a digital copier. A photosensor array is directed toward the photoreceptor to record the actual distribution of toner in response to the creation of test images. As mentioned in the Application, however, there are practical problems with reading toner-based test patterns, especially when trying to use specularly-reflected light in high toner density ranges, to increase the sensitivity of the measurements to spatial variation in toner density.